[LLVMdev] how to get a deterministic execution

[Owen Anderson]

There are a number of places in the optimizers where instructions are held in sets or maps by pointer value, which is non-deterministic.  While the final output has to be the same in all cases, there are several places where iterations are accelerated by using non-deterministic iteration orders.  As long as the same final end result is produced, this is allowed. 

--Owen
 
On Tuesday, May 19, 2009, at 01:47PM, "dan mihai" <dnmh68 at hotmail.com> wrote:
>
>The generated code is deterministic, but the global order the instructions were generated 
>seems it is not (although the result is guaranteed to be the same). 
>Now I think the order functions are processed might not be guaranteed.
>I've attached a test case, and here is what I get:
>...............................
> (303)$ ~/bin/llvm/bin/gcc -O3 -emit-llvm try_calls_basic.2.c -c -o try_calls_basic2.bc
> (304)$  $LLVM/opt  -O3 -IR-DUMP :ALL -load ~/download/llvm-objects/Debug/lib/LLVMHello.so -hello  -debug-pass Structure < try_calls_basic2.bc > try_calls_basic2.2.bc 2> x1
> (305)$  $LLVM/opt  -O3 -IR-DUMP :ALL -load ~/download/llvm-objects/Debug/lib/LLVMHello.so -hello  -debug-pass Structure < try_calls_basic2.bc > try_calls_basic2.2.bc 2> x2
> (306)$ diff x1 x2|more
>237c237
><       %0 = tail call i32 (...)* %pf() nounwind                ; <i32> [#uses=0] {vuid=69}
>---
>>       %0 = tail call i32 (...)* %pf() nounwind                ; <i32> [#uses=0] {vuid=56}
>....................................
>(The -IR-DUMP :ALL is a debugging option I use, that dumps the IR after each function pass  in FPPassManager::runOnFunction)
>
>The vuid are different (69!=56). 
>In fact a closer examination shows that the 56-th Value in the first case is 
>an instruction in main, while in the 2nd case is an instruction in a different function.
>
>My particular issue is that I want to see when the following instruction is created:
>    %pf1.0 = select i1 %0, i32 (...)* bitcast (i32 ()* @pointed_to_function1 to i32 (...)*), i32 (...)* bitcast (i32 ()* @pointed_to_function2 to i32 (...)*)        ; <i32 (...)*> [#uses=2] {vuid=67}
>
>So, I assume it is the 67-th Value, set a conditional breakpoint and restart the program. 
>But this fails, because the second time it is the 55-th value created:
>    %pf1.0 = select i1 %0, i32 (...)* bitcast (i32 ()* @pointed_to_function1 to i32 (...)*), i32 (...)* bitcast (i32 ()* @pointed_to_function2 to i32 (...)*)        ; <i32 (...)*> [#uses=2] {vuid=55}
>
>
>The issues seems to go away, if I remove all other functions from the file and only keep the function I am interested in.
>
>Thank you,
>Daniel
>
>
>> Date: Tue, 19 May 2009 10:59:35 -0700
>> From: isanbard at gmail.com
>> To: llvmdev at cs.uiuc.edu
>> Subject: Re: [LLVMdev] how to get a deterministic execution
>> 
>> On Mon, May 18, 2009 at 8:33 PM, dan mihai <dnmh68 at hotmail.com> wrote:
>> > Hello,
>> >
>> Yo!
>> 
>> > For debugging purposes, I've added a unique id member to the Value class:
>> >
>> > global_next_vuid = 0;
>> > Value::Value(..){
>> >   vuid = ++global_next_vuid;
>> > }
>> >
>> > My hope is that by looking at the vuid of a Value, I can see its vuid,
>> > set a conditional breakpoint and re-run the compiler to
>> > see who (what pass) constructed that value.
>> >
>> > Maybe I am not doing it the right way, but the above 'vuid' field doesn't
>> > seem
>> > to be updated deterministically.
>> >
>> We'd need a testcase to see what you mean...I'm not entirely sure that
>> what you're doing will work, though.
>> 
>> > I am using a DEBUG built. Is LLVM multithreaded by default?
>> >
>> No. LLVM isn't multithreaded safe just yet.
>> 
>> > Is it possible to get a deterministic behavior?
>> >
>> The code that's generated should be completely deterministic.
>> 
>> -bw
>> 
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